This invention relates to fiber optic scanning devices, such as fiber optic image acquisition devices and fiber optic image display devices, and more particularly to a fiber optic scanning device which enhances depth information, achieves a high image resolution and a wide field of view using a flexible fiber of very small diameter.
Fiber optic image acquisition devices include endoscopes, boroscopes and bar code readers. An endoscope is an imaging instrument for viewing the interior of a body canal or hollow organ. Entry typically is through a body opening. A boroscope is an imaging instrument for viewing an internal area of the body. Entry typically is invasive through a xe2x80x98boredxe2x80x99 opening (e.g., a surgical opening).
There are rigid endoscopes and flexible endoscopes. Rigid endoscopes do not have a pixelated image plane. Flexible endoscopes are smaller and conventionally have a pixelated image plane. Flexible endoscopes, however, are unable to achieve the resolution and field of view of rigid endoscopes. But the rigid endoscopes are unable to be used in many applications where small size and flexible fibers and shafts are required.
The goal of any endoscope is high image quality in a small package, allowing minimal tissue trauma. In the growing field of minimally invasive surgical techniques, there is great demand for smaller endoscopes that match current image quality. In particular, the demand for minimally invasive medical procedures has increased the demand for ultrathin optical endoscopes. However, commercial flexible endoscopes have a fundamental tradeoff of size versus image quality. The smaller the endoscope diameter the lower the image resolution and/or field-of-view (FOV), such that image quality deteriorates. Many endoscopic techniques are not possible or become risky when very small endoscopes are used because the doctor has insufficient visual information, i.e. small size and poor quality of images. Accordingly, there is a need for very small, flexible endoscopes with high resolution and FOV. This fundamental tradeoff of a flexible image generator that has both a very small diameter and has the high image quality is a major limitation in applications outside the human body, such as remote sensing.
Conventional flexible endoscopes and boroscopes include a large spatial array of pixel detectors forming a CCD camera. Typically a bundle of optical fibers capture an image and transmit the image to the CCD camera. To achieve a high resolution, wide field image, such CCD cameras often include a pixel detector array of approximately 1000 by 1000 detectors. For color fidelity it is common to include three such arrays, and where stereoscopic viewing is desired, this doubles to six arrays. A fiber is present for each pixel detector. Each fiber has a diameter greater than or equal to 4 microns. Thus, acquisition requires a space of greater than or equal to 4 microns per pixel. If a standard sVGA image is desired (800xc3x97600 pixels), then a minimum diameter of just the image conduit is greater than 3 mm. A 1000 by 1000 pixel detector array has a diameter of at least 4 mm. For a VGA standard, resolution and/or field of view is sacrificed by having fewer pixel elements in order to attain less than 3 mm overall diameter scopes. Reducing the diameter of the endoscope reduces the possible number of pixels, and accordingly, the resolution and field of view. Limits on diameter also limit the opportunity to access color images and stereoscopic images.
In the field of small (e.g., less than 3 mm dia.), flexible endoscopes, the scopes need to use the smallest pixel size, while still reducing the number of pixels, typically to (100xc3x97100). Note, these small flexible endoscopes are found by surgeons to be too fragile, so as not to be widely used. Instead doctors prefer small, but rigid-shafted (straight) endoscopes, greatly limiting their maneuverability and applicability.
In the field of large (e.g., greater than or equal to 4 mm dia.), flexible endoscopes, the scopes have a flexible shaft which is greater than or equal to 4 mm in diameter and typically include either a bundle of optical fibers or a small camera at the distal end to capture the image. However, there is still a tradeoff between the desired 50-70xc2x0 FOV and image resolution at the full potential of human visual acuity until the scope diameter reaches  greater than 10 mm.
U.S. Pat. No. 5,103,497 issued Apr. 7, 1992 of John W. Hicks discloses a flying spot endoscope in which interspacing among fiber optics is decreased to reduce the overall diameter of the optical bundle. Rather than arrange a bundle of fibers in a coherent manner, in his preferred embodiment Hicks uses a multi-fiber whose adjacent cores are phase mismatched. The multi-fiber is scanned along a raster pattern, a spiral pattern, an oscillating pattern or a rotary pattern using an electromagnetic driver. The illumination fibers, the viewing fibers or both the illuminating fibers and the viewing fibers are scanned. In a simplest embodiment, Hicks discloses scanning of a single fiber (e.g., either the illuminating or the viewing fiber).
Hicks uses a small bundle or a single fiber to scan an image plane by scanning the fiber bundle along the image plane. Note that the image plane is not decreased in size. The smaller bundle scans the entire image plane. To do so, the bundle moves over the same area that in prior art was occupied by the larger array of collecting fiber optics. As a result, the area that Hicks device occupies during operation is the same as in prior devices. Further, the core size of the fibers in Hicks"" smaller bundle limits resolution in the same manner that the core size of fibers in the prior larger arrays limited resolution.
One of the challenges in the endoscope art is to reduce the size of the scanning device. As discussed above, the minimal size has been a function of the fiber diameter and the combination of desired resolution and desired field of view. The greater the desired resolution or field of view, the larger the required diameter. The greater the desired resolution for a given field of view, the larger number of fibers required. This restriction has been due to the technique of sampling a small portion of an image plane using a fiber optic camera element. Conventionally, one collecting fiber is used for capturing each pixel of the image plane, although in Hicks one or more fibers scan multiple pixels.
When generating an image plane, an object is illuminated by illuminating fibers. Some of the illuminating light impinges on the object directly. Other illuminating light is scattered either before or after impinging on the object. Light returning (e.g., reflected light, fluorescent returning light, phosphorescent returnig light) from the image plane is collected. Typically, the desired, non-scattered light returning from an illuminated portion of an object is differentiated from the scattered light by using a confocal system. Specifically a lens focuses the light returning to the viewing fiber. Only the light which is not scattered travels along a direct path from the object portion to the lens and the viewing fiber. The lens has its focal length set to focus the non-scattered light onto the tip of the viewing fiber. The scattered light focuses either before or after the viewing fiber tip. Thus, the desired light is captured and distinguished from the undesired light. One shortcoming of this approach is that most of the illuminated light is wasted, or is captured by surrounding pixel elements as noise, with only a small portion returning as the non-scattered light used to define a given pixel.
Minimally invasive medical procedures use endoscopes which present a single camera view to the medical practitioner using a video monitor. The practitioner must mentally relate the flat, two dimensional image captured by the endoscope into the three dimensional geometry of the scanned target within the body. The trained practitioner adapts by using motion parallax, monocular cues and other indirect evidence of depth to mentally envision the geometry of the body. Improving the image presented to the practitioner is desirable. For example, current stereographic endoscopes (with two fiber bundles or cameras) provide additional image data, but exhibit sub-optimal performance. Achieving such improvement without adding substantial cost, weight and size to the endoscope continues to be a challenge.
According to the invention, a miniature image acquisition system having a flexible optical fiber is implemented. The flexible optical fiber serves as an illuminating wave guide which resonates to scan emitted light along a desired pattern. Preferably a single fiber is used for the illumination light. For multiple colors of illumination light, it is preferred that the light from the respective color sources be combined and passed through a distal tip of the single illuminating fiber for emission onto an object being viewed. In alternative embodiments multiple fibers, or concentric fibers, are used for the illumination light.
Rather than generating and sampling an image plane (i.e., in which pixels are spatially separated) as done for conventional flexible endoscopes and the like, an image plane need not be generated to capture an image by the scanner of this invention. Instead pixels are acquired temporally, being separated in time. An advantage of this approach is that image resolution is no longer limited by the detector size (e.g., the diameter of the collecting fiber). According to one aspect of this invention, image resolution, instead, is a function of the illuminating spot size. In particular image resolutions are improved by using a spot size which is smaller than the diameter of the collecting device. In one embodiment single-mode optical fibers are implemented which have smaller gaussian beam profiles and smaller core profiles allowing generation of smaller spot sizes at the scanned site.
Because a pixel is detected as the received light within a window of time, the photons detected at such time window come from the illuminated spot. Another advantage of this invention is that the confocal problem occurring in the prior art systems is avoided. Using a typical video rate, for example, to define the pixel time window sizes, one pixel is collected every 40 nanoseconds. For the light of one pixel to interfere with the light from another pixel, the light of the first pixel would have to bounce around approximately 20 feet on average (because light travels about 1 foot/nanosecond). For typical applications such light would have to bounce around in a space less than one cubic inch. That corresponds to approximately 240 reflections. It is unlikely that the light from one pixel will make 240 reflections before getting absorbed. Thus, the confocal problem is not significant.
According to one aspect of the invention, a distal portion of an illuminating fiber serves as a resonating waveguide. Such distal portion is anchored at an end proximal to the rest of the fiber, (e.g., referred to as the proximal end of the distal portion, or the proximal end of the resonating waveguide). The distal portion is free to deflect and resonate. The waveguide is flexible, being deflected along a desired scan path at a resonant frequency. Light detectors are positioned at the end of the illuminating fiber, (e.g., in the vicinity of the anchored proximal end of the distal portion). Note that collecting fibers may be present, but are not necessary. Further, the detectors may, but need not trace a scan pattern.
An advantage of the invention is that flexibility of the fiber, a wide field of view and high resolution are achieved even for small, thin scopes due to the method in which pixels are obtained, the presence of the lenses and the manner of driving the fiber. Because pixels are measured in time series and not in a 2-D pixel array, it is not mandatory to have small photon detectors. The size of the detector is not critical as in the prior scopes where many small detectors spanned a large area. Therefore, a scope of this invention can be made smaller than existing scopes while using fewer photon detectors that are larger than the pixel detectors of standard scopes. According to the invention, as little as one photon detector may be used for monochrome image acquisition and as few as single red, green and blue detectors may be used for full-color imaging. By adding additional detectors, the advantage of quasi-stereo imaging and photometric stereo is achieved accentuating topography in the full-color images.
According to another aspect of this invention, true stereoscopic viewing is achieved by measuring axial distance from the scope to the target by range finding at each pixel position. Such axial measurement is a third image dimension which is processed to generate stereo views.
According to another advantage of the invention, a single scanning fiber with a small flexible shaft provides (i) axial symmetry, (ii) a low cost method of providing color fidelity, increased object contrast and increased fluorescent contrast, and (iii) laser illumination useful for fluorescent imaging, medical diagnosis and laser surgery. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.